The present invention generally relates to optical modulators and more particularly to an optical modulator suitable for use in an optical telecommunication system that uses optical fibers.
In the long distance optical telecommunication systems that use optical fibers, there arises a problem known as chirping, which is a minute time-dependent fluctuation of wavelength in the modulated optical beams. Such a chirping occurs as a result of self-phase-difference-modulation in the optical fibers, while such a self-phase-difference-modulation occurs each time the optical beam is amplified by optical amplifiers.
With recent tendency of increased modulation frequency that now reaches the order of GHz, the chirping provides a serious effect on the quality of the optical beams transmitted along the optical fibers.
In order to eliminate or minimize the chirping and to improve the quality of the optical beams, it is considered to provide an intentional chirping to the optical beams by way of optical modulators such that the chirping caused in an optical beam as it is propagated through an optical fiber, is effectively canceled out.
Thus, there is a demand for an optical modulator that can control the chirping or wavelength shift as desired.
FIG.1 shows the construction of an optical modulator 10 disclosed in the Japanese Laid-open Patent Publication 4-14010.
Referring to FIG. 1, the optical modulator 10 includes a LiNbO.sub.3 substrate 11 on which an optical waveguide modulator 12 for amplitude modulation and an optical waveguide modulator 13 for phase modulation are provided such that the optical waveguide modulator 13 is cascaded behind the optical waveguide modulator 12.
The optical waveguide modulator 12 includes, on the substrate 11, branched optical waveguides 14 and 15 merged with each other at respective first ends and respective second, opposite ends, wherein a signal electrode 16 is provided on the optical waveguide 14, and a signal electrode 17 is provided on the optical waveguide 15.
The optical waveguide modulator 13, on the other hand, includes an optical waveguide 18 on the substrate 11. Further, a signal electrode 19 and a ground electrode 20 are provided.
Further, the optical modulator 10 of FIG.1 includes signal cables 21 and 22 respectively for supplying electric signals to the foregoing signal electrodes 16 and 19, and a signal source 23 for producing the foregoing electric signals. It should be noted that the electric signal produced by the signal source 23 is supplied directly to the electrode 16 by way of the signal cable 21, while the signal electrode 19 is supplied with the signal from the signal source 23 by way of the signal cable 22, after amplification by an amplifier 24 and processing by an adjustment unit 25. In the illustrated example, a delay element 26 is further included between the adjustment unit 25 and the cable 22. Further, it will be noted that a terminating resistor R.sub.T is provided between the electrode 16 and the electrode 17 and between the electrode 19 and the electrode 20.
In operation, an input optical beam is supplied to a first end of the substrate 11 as indicated in FIG.1 and the optical beam thus supplied is guided along respective paths of the optical waveguides 14 and 15. While propagating through the optical waveguides 14 and 15, the optical beam experiences positive and negative phase modulation in response to the electric signal applied across the electrodes 16 and 17, and these phase differences result in an amplitude modulation at the junction of the two waveguides 14 and 15. Further, the optical beam thus subjected to the amplitude modulation then enters to the optical waveguide 18 and experiences a phase modulation in response to the electric signal applied across the electrodes 19 and 20. The optical beam thus processed is then emitted at a second end of the substrate 11 as indicated.
It should be noted that the optical modulator 10 having such a construction is subjected to an adjustment process, in which the adjustment unit 25 is set such that the optical modulator 10 provides a desired chirp for canceling the chirp of optical beams in the optical fiber. In the adjustment process, the optical waveguide modulators 12 and 13 are operated independently, after the fabrication of the optical modulator 10 is completed, by using an external circuit, while such an adjustment process has to be conducted at the site where the optical modulator 10 is used, in the state that the optical modulator 10 is connected to optical fiber cables. However, such an adjustment process at the site of use of the optical modulators takes a considerable time. Further, it should be noted that the setting of the adjustment unit 25 is fixed once the optical modulator 10 is set to provide the desired chirp, while it is necessary to construct the adjustment unit 25 to allow such an adjustment. Obviously, this is a disadvantage in view of mass production of the optical modulator, and in view of the number of the steps included in the fabrication process. Thus, the conventional optical modulator 10 has suffered from the problem of high cost. Further, the conventional optical modulator 10 has a drawback in the point that the timing adjustment for supplying the electric signals to the optical waveguide modulator 12 and to the optical waveguide modulator 13, is difficult. In addition to the foregoing problems of large number of parts required and expensive construction of the optical modulator, the optical modulator 10 has a drawback in that the construction provided on the substrate 11 for the connection of the external circuit substantially reduces the area used for providing the electrodes 16, 17 or 19, 20. It should be noted that the area of the substrate 11 is substantially limited. As a result, there is a tendency that the electrodes cause an interference on the substrate 11. Further, the reduced separation between the electrodes invites an increased drive voltage of the device.